The recent boom in the photovoltaic (PV) industry has led to an increased demand for polysilicon and silane. One of the challenges in making PV power more commonly available is reducing the cost of the PV module. Since polysilicon production accounts for a significant cost component of the PV module's cost, any improvements in the polysilicon production process will have a significant impact on the PV module cost as well.
Polysilicon is produced from either trichlorosilane (TCS) or silane (SiH4) in a chemical vapor deposition reactor or fluidized bed reactor. Over ninety percent of polysilicon production is by conversion of metallurgical grade silicon (MG-Si) to TCS, which is later purified in a bell jar or fluidized bed reactor in the presence of hydrogen (H2). Some polysilicon is produced by disproportionation of TCS in one or more steps to produce silane (SiH4), which can be used to produce polysilicon or sold separately as a specialty gas for a variety of applications. In either production method, large quantities of silicon tetrachloride (STC) are produced. STC may either be pyrolyzed to high purity fumed silica or, more typically, converted back to TCS in a STC converter. Effluents of the chemical vapor deposition process, the fluidized bed reactor process, and the STC conversion process contain large quantities of hydrogen (H2), chlorosilanes, and hydrochloric acid (HCl).
Effluent gas recovery from the polysilicon and silane production process is an important operation as it can reduce the cost of production. Some have proposed to recycle some of the hydrogen from the effluent gas using a gas separation membrane, such as disclosed by U.S. Pat. No. 4,941,893. However, such a solution comes with certain disadvantages.
When the deposition reactor is used for producing electronic grade polysilicon, it is typically operated at a pressure of about 5 psig. Therefore, the effluent leaving the reactor does not offer much driving force for separation using gas separation membranes. In order to obtain a reasonable recovery, the effluent gas should be compressed prior to feeding it to the membrane, adding a compressor cost.
The deposition reactor used to produce solar grade polysilicon is typically operated at high pressure (>75 psig) and therefore the effluent does not require compression prior to membrane separation. However, the low pressure permeate must be compressed prior to recycle to the deposition reactor pressure, adding a compressor cost.
In either process, the high concentration of chlorosilanes in the effluent may result in loss of membrane permeation properties. This would lead to frequent replacement of the membrane, adding to production cost.
Thus, it is an object to propose a method and system for effluent gas recovery for polysilicon and silane production that avoids the above described disadvantages.